1. Field of the Invention
The present invention relates to the technical field of satellite television transmission and, more particularly, to a fast blind scan method insensitive to adjacent channel interference, which is suitable for a Digital Video Broadcasting-Satellite (DVB-S) or DVB-S2 system.
2. Description of Related Art
For broadly developing digital televisions, the first essential task is to establish a digital platform. In addition to pure hardware equipments such as a set-top box or television set, the digital platform includes digital channels and contents.
Current specifications for global digital televisions are classified essentially into several types: Advanced Television Systems Committee (ATSC) for American-based markets, Digital Video Broadcasting (DVB) for Euro-based markets, Digital Terrestrial Multimedia Broadcasting (DTMB) for China-based markets, and Terrestrial Integrated Services Digital Broadcasting (ISDB-T) for Japan-based markets. The DVB system can be further divided into DVB-C/C2, DVB-S/S2, and DVB-T/T2. The DVB-C/C2 represents the cable television standard. The DVB-S/S2 represents the satellite television standard. The DVB-T/T2 represents the terrestrial wireless television standard.
A central office system provider can transmit a DVB-S/S2 signal to a satellite antenna on a building through a satellite located over 32000 km and above. Such a transmission is applied to send MPEG2 data, and since the position (carrier frequency) and the symbol rate (bandwidth) of each channel on a frequency spectrum are not explicitly defined, the receiver can selectively pre-store the positions (carrier frequencies) and symbol rates (bandwidths) of all known channels in order to rapidly receive the programs.
However, when the carrier frequency and the symbol rate are changed for an updated channel, such a way may not receive the programs on the updated channel smoothly. Accordingly, for such a DVB-S receiver, a blind scan function is a must in order to provide a simple operating mode to thereby allow a user to have an automatic program scan without knowing the programs received by a satellite in advance. The scan rate and the accuracy are the most important indexes for evaluating the performance of the blind scan. In addition, the rapid scan can reduce the waiting time of the user.
FIG. 1 is a block diagram of a typical receiver. As shown, the operation principle is first to use a controller 11 to tune a tuner 12 to a carrier frequency (CF). Next, a filter bank (not shown) and a timing recovery loop (not shown) in a baseband demodulation chip 13 are started to operate with the least symbol rate (SR). The symbol rate is increased when the loop cannot converge. When the symbol rate is over a preset maximum and the loop cannot converge, it indicates that there is no signal at this frequency, and in this case the carrier frequency of the tuner 12 is updated by adding a step to the original basis. Accordingly, the above process is repeated until the loop converges and the signal and associated parameters are detected.
Such a process suffers a disadvantage of very slow scan rate. Since the range of possible symbol rate used by a DVB-S/S2 system is very large, such as from 1M to 45M bauds, and a satellite signal occupies a quite wide spectrum, such as an input range from 950 MHz to 2150 MHz for a Ku or C band tuner, the combination produces numerous possibilities. Accordingly, the blind scan method has to take long time to completely scan the entire possibilities once. In addition, it takes much time to change the carrier frequency of the tuner 12, which significantly prolongs the scan time due to the high step carrier frequencies. Further, the step resolution of the tuner is limited to, for example, hundreds of MHz. Thus, only changing the frequency of the analog tuner cannot accurately detect the carrier frequency. Moreover, the convergence of the loop is very slow, so that it is quite slow to loop-scan the symbol rate. As a result, the typical frequency scan has a very slow rate.
Another blind scan problem to be solved is to reduce the sensitivity of adjacent channel interference (ACI). The magnitudes of adjacent signals in the spectrum are quite different, and can be influenced by other systems or strong signal sources. Accordingly, the only way to achieve the fast and accurate channel scanning target is to reduce the sensitivity of adjacent channel interference. In US patent Publication No. 2011/0135042 filed by the same inventors for a “Blind scan system and method for a DVB-S system”, it has improved the efficiency on the blind scan, but it cannot stand a huge difference on adjacent signal magnitudes.
Therefore, it is desirable to provide an improved fast blind scan method insensitive to adjacent channel interference for DVB-S or DVB-S2 system to mitigate and/or obviate the aforementioned problems.